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 Table of Contents  
Year : 2020  |  Volume : 8  |  Issue : 1  |  Page : 61-63

Pitfalls in diagnosis: Acute hypodense posterior fossa subarachnoid haemorrhage on computed tomography scan – An unusual entity

1 Department of Neuro-Surgery, Rajendra Institute of Medical Sciences, Ranchi, Jharkhand, India
2 Department of Neuro-Surgery, Govind Ballabh Pant Institute of Postgraduate Medical Education and Research, New Delhi, India

Date of Submission06-Aug-2020
Date of Acceptance31-Aug-2020
Date of Web Publication1-Oct-2020

Correspondence Address:
Dr. Gautam Dutta
Department of Neuro-Surgery, Rajendra Institute of Medical Sciences, Ranchi, Jharkhand
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jcvs.jcvs_22_20

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Acute subarachnoid haemorrhage (SAH) generally appears hyperdense on computed tomography (CT) scan. However, in some rare circumstances, it may appear hypodense which can pose a diagnostic problem and compromise patient care. We report the case of a 32-year-old man who had presented with acute posterior fossa SAH. A cranial CT scan revealed a hypodense posterior fossa lesion mimicking cerebellar infarct. Surgical intervention confirmed that it was an acute bleed in the subarachnoid space. Treating physicians should be cautious in interpreting CT images as that can sometimes be misleading leading to misdiagnosis and incorrect treatment choices.

Keywords: Computed scan, hypodensity, infarct, subarachnoid haemorrhage

How to cite this article:
Dutta G, Singh D, Jagetia A, Srivastava AK. Pitfalls in diagnosis: Acute hypodense posterior fossa subarachnoid haemorrhage on computed tomography scan – An unusual entity. J Cerebrovasc Sci 2020;8:61-3

How to cite this URL:
Dutta G, Singh D, Jagetia A, Srivastava AK. Pitfalls in diagnosis: Acute hypodense posterior fossa subarachnoid haemorrhage on computed tomography scan – An unusual entity. J Cerebrovasc Sci [serial online] 2020 [cited 2023 Feb 4];8:61-3. Available from: http://www.jcvs.in/text.asp?2020/8/1/61/296929

  Introduction Top

In an emergency set-up, computed tomography (CT) of the brain is the most important first imaging investigation obtained in patients presenting with new-onset sudden, severe headaches to rule out subarachnoid haemorrhage (SAH). CT, beyond doubt, is a sensitive test for detecting subarachnoid blood, but in some rare circumstances, CT images can misinterpret blood in subarachnoid spaces. We present a case of posterior fossa subarachnoid haemorrhage which CT misdiagnosed as a hypodense lesion. This finding could pose a diagnostic problem and compromise patient care. We discuss the possible pathophysiology of this phenomenon.

  Case Report Top

A 32-years-old male patient was referred to our emergency department with complaints of sudden-onset severe occipital headache with multiple episodes of vomiting and disoriented behaviour for the past 1 day. There was no history of trauma or any significant medical history or any history of drug abuse. On examination, his general condition was very poor with a pulse rate of 54/min, blood pressure 100/50 mm Hg, respiratory rate 24/min and blood oxygen of 80%. On neurological examination, he was drowsy, pupils were intermediate and sluggishly reacting to light. The neck rigidity was present on examination. He opened eyes only to painful stimulus and showed spontaneous decerebration. Immediate orotracheal intubation was done and mechanical ventilation started. His complete blood counts, renal and liver function tests, electrolytes and tests of coagulation were all within the normal limits. CT of the brain was obtained, which revealed a zone of hypodensity involving the right cerebellar hemisphere with significant mass effect and heterogeneous hyperdensity involving the left cerebellar hemisphere [Figure 1]. Based on these findings, a possible diagnosis of acute right cerebellar infarct was performed. Magnetic resonance imaging (MRI) and magnetic resonance angiography of the brain to exclude aneurysms or arteriovenous malformations were planned but could not be performed due to his deteriorating status.
Figure 1: Non-contrast computed tomography of the brain showing zone of hypodensity involving the right cerebellar hemisphere with significant mass effect and heterogeneous hyperdensity involving the left cerebellar hemisphere

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With CT findings of significant mass effect and clinical signs of raised intracranial pressure, surgical intervention was planned. He was urgently shifted to the operating room and a suboccipital decompressive craniectomy was performed in the prone position. Dura was found to be tense and the brain was bulging. Dark blood could be appreciated underlying the dura mater. After the dura was exposed, blood clots were seen below the arachnoid layer of both cerebellar hemispheres [Figure 2]a. To evacuate the haematoma, the incision was extended giving access to the entire posterior fossa and confirming the clotted appearance of the acute SAH. The arachnoid was opened and ~50 ml clotted blood was evacuated. The cerebellum was examined, which was found to be healthy without any evidence of infarction [Figure 2]b. Cerebellar pulsations, which was absent initially gradually returned following decompression. The tonsils were elevated to assess the cerebrospinal fluid (CSF) flow, which was found to be adequate.
Figure 2: Intraoperative pictures: (a) clotted blood below the arachnoid layer of both cerebellar hemispheres; (b) arachnoid has been cut and blood clots evacuated revealing normal-appearing cerebellum

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Post-operatively, the patient's condition remained critical and he was admitted to our neurosurgical intensive care and kept on ventilatory support. He remained hypotensive, tachycardic and oliguric. Injection noradrenaline (2 μg/min) and injection dopamine (16 μg/kg/min) was started to keep mean arterial pressure above 65 mm Hg. Electrocardiogram was obtained, which showed sinus tachycardia at a rate of 120 beats/min. A post-operative CT brain was obtained, which revealed the previously ”hypodense area” initially thought to be the infarcted core, has undergone complete resolution [Figure 3]. In view of possible vertebrobasilar system aneurysm rupture/dissection, cerebral angiography was planned but could not be performed due to his critically ill condition. The patient's condition remained critical and he died on the 4th post-operative day despite aggressive measures, possibly due to the sequelae of the brainstem compression with cardiac and respiratory depression.
Figure 3: Post-operative non-contrast computed tomography of the brain showing evacuation of haematoma with a resolution of the previously hypodense area

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  Discussion Top

SAH is the extravasation of the blood in the subarachnoid space and happens when there is a rupture of the vessels traversing through the subarachnoid space above the brain parenchyma. This bleed is seen as irregular hyperdensity in the subarachnoid space on CT. Unenhanced CT is commonly used in an emergency room setting for acute stroke because of its convenience and its high sensitivity for detecting intracranial haemorrhage (ICH), which is a contraindication to thrombolytic therapy.[1] Moreover, CT allows to quantify haematoma volume and monitor haemorrhage evolution in ICH accurately.[2]

CT scan is also able to determine the approximate age of haematomas, by evaluating for the density of the lesions measured in Hounsfield units (HUs), according to the value of X-ray attenuation corrected for the attenuation coefficient of water. An acute bleed usually appears hyperdense on the CT scan and hyperdensity is related to coagulated blood (density of 60–80 HU), which then decreases by about 1.5 HU/day, leading eventually to a return to isodensity with the brain parenchyma in 10–20 days.[3] In the hyperacute stage, blood sometimes appears heterogeneous with hyperdense and isodense images. This is related to the presence of non-coagulated blood in connection with bleeding disorders or active bleeding. A homogeneous hypodensity is most unusual in acute settings.[3]

Lee et al. had studied of 351 patients with acute subdural haematoma where blood appeared hyperdense in 98.6% of cases, isodense in 1.1% and hypodense in 0.3% of the cases.[4] The acute nature was defined by a delay between the trauma and the scan of <7 days.

In a small proportion of cases, an acute bleed may appear isodense or even hypodense compared with the adjacent parenchyma. This situation is encountered in cases of anaemia, disseminated intravascular coagulation, or if the haematoma is diluted with CSF, as noted in another study.[5] Other authors have reported a case of an intracerebral haematoma that appeared isodense in a patient with disseminated intravascular coagulation.[6]

Most cases in the literature pertaining to hypo/isodense CT images in acute bleed are described in the setting of subdural or extradural haemorrhages. Despite extensive Medline search, we could not find any article that studied hypodense SAH on CT in acute settings. In our case, the likely reason behind the hypodense appearance of SAH in the right cerebellar hemisphere was the CSF dilution of blood. The local difference in fibrinolytic activity, septation of haematoma and fresh haemorrhage might have produced the mixed intense appearance of SAH in the left cerebellar hemisphere. However, further studies are required to explore this although rare, variable appearance of acute blood, especially in the setting of SAH.

  Conclusion Top

The misdiagnosis of infarct by CT of the brain in patients with a suggestive clinical presentation of SAH may delay the diagnosis of the true underlying pathology. It is appropriate to consider alternative diagnoses, especially when there is a clinical-radiological picture mismatch. MRI of the brain with diffusion imaging may help differentiate, though getting an MRI in a critically ill patient may not always be feasible. When in doubt in front of an equivocal image, the patient's condition and clinical history should prevail in the therapeutic decision. In instances of doubt and in deteriorating clinical condition of the patient, urgent surgical intervention should be considered inappropriate cases, as no imaging modality can indeed compare the adequacy of direct visual inspection by the neurosurgeon.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given his consent for his images and other clinical information to be reported in the journal. The patient understand that name and initials will not be published and due efforts will be made to conceal identity, but anonymity cannot be guaranteed.

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Conflicts of interest

There are no conflicts of interest.

  References Top

Morgenstern LB, Hemphill JC 3rd, Anderson C, Becker K, Broderick JP, Connolly ES Jr., et al. American heart Association Stroke Council and Council on Cardiovascular Nursing. Guidelines for the management of spontaneous intracerebral hemorrhage: A guideline for healthcare professionals from the American Heart Association/American Stroke Association. Stroke 2010;41:2108-29.  Back to cited text no. 1
Zimmerman RD, Maldjian JA, Brun NC, Horvath B, Skolnick BE. Radiologic estimation of hematoma volume in intracerebral hemorrhage trial by CT scan. AJNR Am J Neuroradiol 2006;27:666-70.  Back to cited text no. 2
Grelat M, Madkouri R, Bousquet O. Acute isodense subdural hematoma on computed tomography scan--diagnostic and therapeutic trap: a case report. J Med Case Rep 2016;10:43.  Back to cited text no. 3
Lee KS, Bae WK, Bae HG, Doh JW, Yun IG. The computed tomographic attenuation and the age of subdural hematomas. J Korean Med Sci 1997;12:353-9.  Back to cited text no. 4
Deb S, Bhaumik S, Pal H. Isodense acute subdural haematoma in anaemic patients. Neurol India 2000;48:298-9.  Back to cited text no. 5
[PUBMED]  [Full text]  
Claes F, Verhagen CV, Verhagen WI, Schaafsma E, Rongen RJ. Acute isodense intracerebral haematoma due to coagulopathy associated with prostate cancer. Clin Neurol Neurosurg 2007;109:520-2.  Back to cited text no. 6


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